It is known in the art to use a capacitance probe to measure the dielectric constant of a liquid that occupies the space between plates of the probe. An alternating current signal is provided wherein the magnitude of the signal is proportional to the probe capacitance and thus to the value of the dielectric constant of the liquid. This alternating current signal is then processed with suitable circuitry to give an output signal representative of the liquid density.
U.S. Pat. No. 4,011,746, issued to Weit on Mar. 15, 1977, teaches a liquid density measurement system including such a capacitance probe. The probe is mounted for immersion in a mass of liquid, the density of which is to be measured. The capacitance of the probe depends on the dielectric constant of the liquid. A capacitance converter unit is connected to the probe and arranged to produce a signal which is dependent on the dielectric constant of the liquid. The system of Weitz is further provided with a temperature sensor also mounted for immersion in the liquid. A temperature signal conditioning unit is connected to the temperature sensor and adapted to provide a signal dependent upon the temperature of the liquid. A computation unit receives the two signals to produce an output signal proportional to the density of the liquid. Thus it is known in the art to compensate for temperature variation when using a capacitance probe to determine the dielectric constant of a liquid.
Additionally, the determination of dielectric constants of more viscous fluids is known. Some fluids may undergo a change in their dielectric properties as their viscosity changes thereby allowing a process accompanied by such changes to be monitored by capacitance probes. For example, dielectric properties are used in standard cure monitoring procedures and can be measured inside operating autoclaves. However, no adequate theoretical relationship has been determined between dielectric properties and rheological properties and viscometric properties. Such a theoretical relationship could be used for process control because the dielectric properties can be satisfactorily monitored during the cure process.
Thus it is difficult to determine the relationship between the deformability of the substance being cured or the ability of the substance being cured to flow, and the dielectric constant as measured by a capacitance probe. This is particularly true of multiphase systems where the proportion of material in each phase varies with time. In some pre-preg systems and in many adhesives, the matrix changes phase from liquid to gel to glass. Additionally, rubber particles may be precipitated during cure. Powder and fibrous fillers may be present. These fillers may flow with the matrix. Additionally, reinforcing fibers may be present. These reinforcing fibers remain stationary during resin flow. It is necessary to determine the viscosity profile of a resin being cured in a way which relates the viscosity profile directly to a dielectric profile representing capacitance and loss measurements. It is preferable that both measurements be determined simultaneously.
It is therefore an object of this invention to develop an instrument capable of accurate determination of the absolute viscosity of resin matrix materials and adhesives and its variation with time during resin cure cycles.
In particular, it is an object of the invention to monitor viscosity of isolated matrix resin, or unfilled adhesive.
A further object of the invention is to monitor the absolute or relative viscosity of matrix resin incorporated into a fiber bed or into pre-preg material and fiber-filled adhesive.